Remove flank steak from freezer and slice on the bias against the grain into 1/4-inch thick pieces. Heat 1 tablespoon of oil in a cast iron griddle or skillet, and saute the red peppers until they blacken around the edges, and soften, about 8 minutes. Remove from griddle and turn heat to medium-high, add more oil if necessary and sear steak in batches, about 2 minutes.

To assemble the sandwiches: Cut bread loaves in half, leaving 1 side attached. Layer on avocado slices, top with flank steak, red pepper, and cilantro. Drizzle with chipotle cheese sauce and serve immediately.